The invention relates to a twist lock attachment system for attaching a fixture, such as a lighting fixture, to a structural support such as a wiring conduit. The invention particularly applies to lighting fixtures having sheet metal housings.
Fixtures, for example commercial and industrial lighting fixtures, are often installed by hanging from a structural support such as a threaded conduit. A commercial or industrial lighting fixture that includes a ballast transformer can have a substantial weight, making the installation task difficult.
Thus, hanging a heavy, sheet metal housed lighting fixture from a conduit typically requires either extensive bracketry, or expensive permanently-attached threaded bushings. As an example of bracketry, some lighting fixture designs include a relatively small cast plate-like mounting bracket that threads onto the conduit. The heavier, main part of the lighting fixture then slides onto the mounting bracket plate. With permanently-attached bushings, an installer must twist a heavy lighting fixture onto a threaded conduit without damaging the threads or dropping the fixture.
In the case of lighting fixtures that have a die cast housing, in contrast to a sheet metal housing, a current practice is to include a twist lock bushing as part of the housing. Thus, integral hook-like tabs are cast into the housing, and cooperate with a separate locking ring that threads on to the conduit. Twist lock bushings are particularly convenient to use. Mating elements of a twist lock bushing assembly are engaged and then rotated relative to each other a relatively short angular distance, such as 90xc2x0, which is far simpler for an installer compared to the multiple rotations required to attach a conventional threaded bushing to a conduit. However, a die cast housing with an integral twist lock bushing element is more expensive than a sheet metal housing.
It is therefore seen to be desirable to provide a twist lock attachment system suitable for use with a fixture, such as a lighting fixture, having a housing made of sheet metal.
In an exemplary embodiment, a twist lock attachment system includes, in general, a mounting plate made of sheet metal included as part of a fixture, as well as a twist lock bushing attachable to a structural support, such as a threaded conduit. The mounting plate has an exterior surface and an opposed interior surface separated by a mounting plate thickness, as well as a generally circular mounting aperture. The twist lock bushing includes a tubular body correspondingly generally circular in cross section, and having an insertion end and an opposite end. The mounting plate and the twist lock bushing releasably engage each other by relative movement to insert the tubular body into the mounting aperture with the mounting plate and the bushing angularly oriented in an insertion and removal position with reference to each other, and subsequent rotation of the mounting plate and the twist lock bushing relative to each other in a first rotational direction to an installed position.
The tubular body more particularly includes a radially-extending top flange located intermediate the opposite end and the insertion end. The top flange has a flange bearing surface axially facing the insertion end for limiting relative axial movement in an insertion direction by bearing against the exterior surface of the mounting plate. The flange bearing surface accordingly defines a flange bearing surface plane. The tubular body additionally includes at least two radially-extending locking tabs intermediate the top flange and the insertion end generally adjacent the insertion end. The locking tabs have locking tab bearing surfaces axially facing the opposite end for retaining the tubular body within the mounting aperture by bearing against the interior surface of the mounting plate in the installed position. Thus, the tab bearing surfaces are separated from the flange bearing surface plane a distance corresponding to the thickness of the mounting plate. For clearing the locking tabs as the tubular body is inserted into or removed from the mounted aperture in the insertion and removal position, the mounting plate has at least two locking tab clearance slots extending radially from the mounting aperture. Releasable elements are provided for preventing relative rotational movement of the mounting plate and the bushing in the installed position.